The specificity of certain microorganisms or of certain enzymes derived from microorganisms enables their potential use for the preparation of enantiomerically pure intermediates from racemic mixtures. The desired enantiomeric molecule can then be transformed into the target compound. Microorganism or enzyme-catalyzed resolution of isomers offers an attractive alternative to more traditional and costly methods, such as chemical resolution and high performance liquid chromatography of diastereomeric derivatives.
Kato et al. have reported that a known bacterium, Corynbacterium equi IFO 3730, has the ability to hydrolyze various esters enantioselectively (Tetrahedron Letters, Vol. 28, No. 12, 1987, pages 1303-1306). In their study, the microorganism was applied to the asymmetric hydrolysis of 2-benzyloxy substituted alkane- and arylalkane carboxylic acid esters, using a suspension of grown cells of C. equi and a prolonged (e.g., 24 hours) fermentation process. Unreacted lower alkyl esters were recovered in the optically active S-form in high enantiomeric excess (over 99% e.e.). It was also found that changing the alkyl or alkenyl moiety of the substrate with a phenylmethyl group caused a reversal of stereoselectivity, resulting in recovery of the optically active R-form, also in high enantiomeric excess.
Kitazume et al. have described a procedure for the asymmetric hydrolysis of 2-fluoro-2-methylmalonic acid diesters with pig liver esterase, giving the optically active (-)-2-fluoro-2-methylmalonic acid monoesters but with low enantiomeric excess. Also reported were the microbial hydrolysis of 2-fluoro-2-substituted malonic acid diesters with both esterase and cellulase to give the optically active (+)- or (-)-2-fluoro-2-substituted malonic acid monoesters (J. Org. Chem. 51, 1986, pages 1003-1006).
Gu et al. have reported that optically active 3-benzoylthio-2-methylpropionic acids can be prepared through the microbial lipase-catalyzed enantioselective hydrolysis of their corresponding esters. Enantio-selectivity to the desired sterochemically preferred S-isomer was poor with all lipases tried, necessitating structural changes in the aroylthio moiety of the substrate compound to achieve higher stereoselectivity. In particular, introduction of methoxy groups into the phenyl ring at the 3 and 5 positions resulted in improved stereospecificity using the lipase of Mucor meihei. (Tetrahedron Letters, Vol. 27, No. 43, 1986, pages 5203-5206).
Iuchijima et al. have described a process for the production of optically active 2-chloro- and 2-bromo-substituted alkyl esters and acids by the asymmetric hydrolysis of racemic mixtures of the ester, using the microorganisms Rizopus, Mucor, Aspergillus, Candida, Pseudomonas, Alcaligenes, Achromobacter and Bacillus, or enzymes derived from them. Published Japan Patent Application (Kokai) No. 57-94,295 (1982).
Also reported in the literature have been the Candida lipase-catalyzed enantioselective hydrolysis of racemic octyl 2-chloropropionate to the R-form of 2-chloropropionic acid (Cambou and Klibanov, Appl. Biochem. Biotech, 9, 1984, Page 255).
U.S. Pat. No. 4,668,628 (Dahod et al.) discloses a process for enzymatically resolving racemic mixtures of partially water-soluble esters, which involves contacting the racemic mixture with a Candida lipase enzyme to enzymatically hydrolyze it. A specific example is the Candida lipase catalyzed hydrolysis of D,L-methyl-2-chloropropionate.
A disadvantage of lipase-catalyzed kinetic resolutions in particular is that the specificity of the enzyme for a given substrate often cannot be anticipated in advance, since there is no useful model available for predicting the stereochemical outcome of a lipase-catalyzed kinetic resolution of a potential substrate.